One contribution of 10 to a theme issue 'Super-sensing through industrial process tomography' . The contactless inductive flow tomography (CIFT) is a measurement technique that allows reconstructing the flow of electrically conducting fluids by measuring the flow-induced perturbations of one or various applied magnetic fields and solving the underlying inverse problem. One of the most promising application fields of CIFT is the continuous casting of steel, for which the online monitoring of the flow in the mould would be highly desirable. In previous experiments at a smallscale model of continuous casting, CIFT has been applied to various industrially relevant problems, including the sudden changes of flow structures in case of argon injection and the influence of a magnetic stirrer at the submerged entry nozzle. The application of CIFT in the presence of electromagnetic brakes, which are widely used to stabilize the flow in the mould, has turned out to be more challenging due to the extreme dynamic range between the strong applied brake field and the weak flow-induced perturbations of the measuring field. In this paper, we present a gradiometric version of CIFT, relying on gradiometric field measurements, that is capable to overcome those problems and which seems, therefore, a promising candidate for applying CIFT in the steel casting industry.This article is part of the themed issue 'Supersensing through industrial process tomography'.

No abstract

The contactless inductive flow tomography (CIFT) aims at reconstructing the velocity field of electrically conducting fluids, with special focus on applications in metallurgy and crystal growth technologies. The method relies on the induction of secondary magnetic fields if the moving fluid is exposed to a primary magnetic field. The theoretical foundation of the method is delineated, and some early experiments on the reconstruction of the three-dimensional flow in a cylinder are sketched. Then, the recent efforts to apply CIFT to various model problems in connection with the continuous casting of steel are summarized.

For contactless inductive flow tomography we require a compact magnetic field measurement system with a dynamic range of 5 orders of magnitude in order to detect the amplitude and the phase of an alternating magnetic field of 1 mT strength with a precision better than 5 nT and a phase error no larger than 10−2 deg. In some applications a static magnetic field of about 300 mT is also present, resulting in a total dynamic range of 7 orders of magnitude. We present theoretical and experimental analyses of absolute and first order gradiometric induction coil sensors with sensitivities larger than 500 V/T · Hz) and diameters of 28 mm. From their equivalent circuits, we derive the associated complex-valued transfer functions and fit these to calibration measurements, thereby determining the value of the equivalent circuit components. This allows us to compensate their non-linear frequency-dependent amplitude and phase behaviour. Furthermore, we demonstrate the optimization of coils based on Brooks’ design of equal squares in the adaptation by Murgatroyd, which maximizes the inductance (and thereby most likely the sensitivity) of the coils. Finally, we design a new coil with a diameter of 74 mm and a sensitivity of 577 V/(T · Hz) with an analytically predicted equivalent magnetic field noise of around in the 1 Hz frequency range, which is then confirmed by measurements on the manufactured prototype.

No abstract